Universal mobile telecommunications system (UMTS) is a 3rd Generation (3G) asynchronous mobile communication system operating in wideband code division multiple access (WCDMA) based on European systems, global system for mobile communications (GSM) and general packet radio services (GPRS). The long-term evolution (LTE) of UMTS is under discussion by the 3rd generation partnership project (3GPP) that standardized UMTS.
The 3GPP LTE is a technology for enabling high-speed packet communications. Many schemes have been proposed for the LTE objective including those that aim to reduce user and provider costs, improve service quality, and expand and improve coverage and system capacity. The 3G LTE requires reduced cost per bit, increased service availability, flexible use of a frequency band, a simple structure, an open interface, and adequate power consumption of a terminal as an upper-level requirement.
The 3GPP LTE uses an orthogonal frequency division multiple access (OFDMA) on a lower link, and uses a single carrier-frequency division multiple access (SC-FDMA) on an upper link. And, the 3GPP LTE adopts a multiple input multiple output (MIMO) system having maximum four antennas. Recently, research on 3GPP LTE-A (LTE-Advanced), an advanced system of the 3GPP LTE is being actively performed.
Techniques applied to the 3GPP LTE-A include carrier aggregation, relay, etc. The 3GPP LTE system is a single carrier system for supporting one bandwidth among ‘1.4, 3, 5, 10, 15 and 20’ MHz, i.e., one component carrier. On the other hand, the LTE-A adopts a multiple carrier using carrier aggregation. The component carrier is defined as a center frequency and a bandwidth. In a multiple carrier system, a plurality of component carriers having a bandwidth narrower than an entire bandwidth are used.
FIG. 1 is a block diagram illustrating network structure of an evolved universal mobile telecommunication system (E-UMTS). The E-UMTS may be also referred to as an LTE system. The communication network is widely deployed to provide a variety of communication services such as voice and packet data.
As illustrated in FIG. 1, the E-UMTS network includes an evolved UMTS terrestrial radio access network (E-UTRAN), an Evolved Packet Core (EPC) and one or more user equipment. The E-UTRAN may include one or more evolved NodeB (eNodeB) 20, and a plurality of user equipment (UE) 10 may be located in one cell. One or more E-UTRAN mobility management entity (MME)/system architecture evolution (SAE) gateways 30 may be positioned at the end of the network and connected to an external network.
As used herein, “downlink” refers to communication from eNodeB 20 to UE 10, and “uplink” refers to communication from the UE to the eNodeB 20. The UE 10 refers to communication equipment carried by a user and may be also be referred to as a mobile station (MS), a user terminal (UT), a subscriber station (SS) or a wireless device.
The eNodeB 20 provides end points of a user plane and a control plane to the UE 10. MME/SAE gateway 30 provides an end point of a session and mobility management function for the UE 10. The eNodeB and MME/SAE gateway may be connected via an S1 interface.
The eNodeB 20 is generally a fixed station that communicates with a UE 10, and may also be referred to as a base station (BS) or an access point. One eNodeB 20 may be deployed per cell. An interface for transmitting user traffic or control traffic may be used between eNodeBs 20.
The MME provides various functions including distribution of paging messages to eNodeBs 20, security control, idle state mobility control, SAE bearer control, and ciphering and integrity protection of non-access stratum (NAS) signaling. The SAE gateway host provides assorted functions including termination of U-plane packets for paging reasons, and switching of the U-plane to support UE mobility. For clarity MME/SAE gateway 30 will be referred to herein simply as a “gateway,” but it is understood that this entity includes both an MME and an SAE gateway.
A plurality of nodes may be connected between eNodeB 20 and gateway 30 via the S1 interface. The eNodeBs 20 may be connected to each other via an X2 interface and neighboring eNodeBs may have a meshed network structure that has the X2 interface.
FIG. 2 is a block diagram illustrating a wireless protocol structure with respect to a user plane, and FIG. 3 is a block diagram illustrating a wireless protocol structure with respect to a control plane. A data plane indicates a protocol stack for user data transmission, and a control plane indicates a protocol stack for control signal transmission.
Referring to FIGS. 2 and 3, a physical layer provides an information transmission service to an upper layer by using a physical channel. The physical layer is connected with a medium access control (MAC) layer located at a higher level through a transport channel, and data between the MAC layer and the physical layer is transferred via the transport channel. The transport channel is classified according to a data transfer type in a specific manner through a radio interface.
Data is transferred through the physical channel between different physical layers, i.e., a physical layer of a transmitter and a physical layer of a receiver. The physical channel is modulated in an Orthogonal Frequency Division Multiplexing (OFDM) manner, and utilizes time and a frequency as radio resources.
Functions of the MAC layer include mapping between a logical channel and a transport channel, and multiplexing/de-multiplexing of a MAC service data unit (SDU) which belongs to the logical channel into a transport block provided to the physical channel on the transport channel. The MAC layer provides a service to a Radio Link Control (RLC) via a logical channel.
The RLC layer performs concatenation, segmentation and reassembly of an RLC SDU. In order to ensure various qualities of service (QoS) requested by a radio bearer (RB), the RLC layer provides three operation modes including a transparent mode (TM), an unacknowledged mode (UM) and an acknowledged mode (AM). The AM RLC provides error correction through an automatic repeat request (ARID).
Functions of a Packet Data Convergence Protocol (PDCP) layer on the user plane include user data transfer, header compression, and ciphering. And, the function of a Packet Data Convergence Protocol (PDCP) layer on the user plane includes transfer of control plane data, and ciphering/
integrity protection.
A Radio Resource Control (RRC) layer is defined only on the control plane. The RRC layer controls a logical channel, a transport channel, and a physical channel with respect to configuration, re-configuration, and release of radio bearers (RBs). The RB indicates a logical path provided by a first layer (PHY layer) and, a second layer (MAC layer, RLC layer and PDCP layer) for data transfer between a UE and a network. RB establishment indicates that a radio protocol layer and a channel characteristic are defined and each parameter and operation are configured so as to provide a specific service. The RB may be divided into a Signaling RB (SRB) and a Data RB (DRB). The SRB is used as a path for transferring an RRC message on a control plane, whereas the DRB is used as a path for transferring a user data on a user plane.
When RRC connection is implemented between an RRC layer of a UE and an RRC layer of an E-UTRAN, the UE is in an RRC_CONNECTED state. Otherwise, the UE is in an RRC_IDLE state.
A downlink transport channel through which data is transferred to a UE from a network includes a Broadcast Channel (BCH) for transferring system information, and a downlink Shared Channel (SCH) for transferring user traffic or a control message. Traffic or a control message of a downlink multicast or a broadcast service may be transferred through the downlink SCH, or through an additional downlink Multicast Channel (MCH). An uplink transport channel through which data is transferred from the UE to the network includes a Random Access Channel (RACH) for transferring an initial control message, and an uplink SCH for transferring user traffic or a control message.
Logical channels which are upper layers of transportation channels and mapped to the transportation channels include a Broadcast Control Channel (BCCH), a Paging Control Channel (PCCH), a Common Control Channel (CCCH), a Multicast Control Channel (MCCH), a Multicast Traffic Channel (MTCH), etc.
The physical channel consists of a plurality of symbols in time, and a plurality of sub-carriers in frequency. One sub-frame consists of a plurality of symbols in time. One sub-frame consists of a plurality of resource blocks, and one resource block consists of a plurality of symbols and sub-carriers. Each sub-frame may use specific sub-carriers of specific symbols (e.g., first symbol) of a corresponding sub-frame for a Physical Downlink Control Channel (PDCCH), i.e., an L1/L2 control channel. A transmission time interval (TTI), a unitary time during which data is transferred is ‘1 ms’ corresponding to one sub-frame.
Hereinafter, an RRC state and an RRC connection of a UE will be explained in more detail.
The RRC state indicates whether an RRC layer of a UE is logically connected with an RRC layer of an E-UTRAN or not. If the RRC layer of the UE is logically connected with the RRC layer of the E-UTRAN, the state is referred to as an RRC_CONNECTED state). On the other hand, if the RRC layer of the UE is not logically connected with the RRC layer of the E-UTRAN, the state is referred to as an RRC_IDLE state. The UE in the RRC_CONNECTED state has an RRC connection. Accordingly, the E-UTRAN can check whether the corresponding UE exists or not as a cell unit, and thus effectively control the UE. On the other hand, the UE in the RRC_IDLE state can not be checked by the E-UTRAN, but can be managed by a core network as a tracking area unit, a unit of a region larger than a cell. More concretely, whether the UE in the RRC_IDLE state exists or not can be checked only as a large area unit, and the UE in the RRC_IDLE state has to transit to the RRC_CONNECTED state so as to be provided with a general mobile communications service such as voice or data.
When a user turns on a UE, the UE searches a suitable cell and then is in an RRC_IDLE state in the corresponding cell. The UE in the RRC_IDLE state is RRC-connected with the E-UTRAN when necessary through an RRC connection procedure, and transits to the RRC_CONNECTED state. The UE in the RRC_IDLE state is required to have RRC connection in the following cases. For instance, when uplink data transfer due to a user's call attempt is required, or when message transmission in response to a paging message from the E-UTRAN is required, the RRC connection is executed.
A Non-Access Stratum (NAS) layer, an upper layer of the RRC layer executes functions such as session management and mobility management.
For the UE's mobility management by the NAS layer, an EPS Mobility Management-REGISTERED (EMM-REGISTERED) state and an EMM-DEREGISTERED state are defined. These two states are applied to the UE and an MME. The initial UE is in an EMM-DEREGISTERED state, and registers, through an initial attach procedure, to a network for connection to the network. Once the attach procedure is successfully executed, the UE and the MME are in the EMM-REGISTERED state.
For management of signaling connection between the UE and an EPC, an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. These two states are applied to the UE and the MME. When the UE in the ECM-IDLE state has RRC connection with the E-UTRAN, the UE is in the ECM-CONNECTED state. When the MME in the ECM-IDLE state has S1 connection with the E-UTRAN, the MME is in the ECM-CONNECTED state. When the UE is in the ECM-IDLE state, the E-UTRAN does not have context information of the UE. Accordingly, the UE in the ECM-IDLE state executes a UE-based mobility procedure such as cell selection or reselection without being instructed by a network. On the other hand, when the UE is in the ECM-CONNECTED state, the UE's mobility is managed by a command from the network. If a current position of the UE in the ECM-IDLE state is different from a position recognized by the network, the UE informs its position to the network through a tracking area update procedure.
Hereinafter, radio link failure will be explained.
The UE continuously executes measurements so as to maintain quality of a radio link with a serving cell which is receiving a service. The UE determines whether the current state is a service impossible state due to quality deterioration of the radio link with the serving cell. If the current quality of the serving cell is too deteriorated to implement communications, the UE determines the current state as ‘radio link failure’.
If the current state is determined as ‘radio link failure’, the UE does not maintain the communicated state with the serving cell any longer, but selects a new cell through a cell selection (or reselection) procedure. Then, the UE tries an RRC connection re-establishment procedure to the new cell.
FIG. 4 is an exemplary view illustrating radio link failure. Operations relating to the radio link failure may be described according to two phases.
According to the first phase, the UE is in a normal operation state, and checks whether a problem has occurred on a current communication link. If the occurrence of a problem is detected, the UE determines that the current communication link is in a radio link problem, and waits for recovery of the radio link during a first standby time (T1). If the radio link is recovered before the first standby time lapses, the UE executes a normal operation again. On the other hand, if the radio link is not recovered until the first standby time is expired, the UE determines that radio link failure has occurred and enters the second phase.
According to the second phase, the UE attempt to re-establish RRC connection by performing re-establishment procedure for recovery of the radio link during a second standby time (T2). If the radio link is not recovered until the second standby time is expired, the UE enters an RRC_IDLE state.
The RRC connection re-establishment procedure indicates a procedure of re-establishing RRC connection in an RRC_CONNECTED state. Since the UE does not enter an RRC_IDLE state, the UE does not initialize any connection establishments such as radio bearer establishment. Rather, the UE temporarily suspends usage of other radio bearers except for an SRB when starting an RC connection re-establishment procedure. If the RRC connection re-establishment procedure is successful, the UE resumes the suspended usage of the radio bears.
Hereinafter, a multiple carrier system will be explained.
A 3GPP LTE system supports a case where a downlink bandwidth and an uplink bandwidth are different configured. This is implemented under an assumption of one component carrier (CC). The CC is defined as a center frequency and a bandwidth. This means that the 3GPP LTE system is supported only in a case that a downlink bandwidth and an uplink bandwidth are equal to each other or different from each other, in a state that one CC is defined with respect to each of an upper link and a downlink. For instance, the 3GPP LTE system supports 20 MHz to the maximum, and supports one CC to each of an uplink and a downlink even if an uplink bandwidth and a downlink bandwidth may be different from each other.
One CC may correspond to one cell. A carrier frequency corresponds to a center frequency of a CC, or a center frequency of a cell. Accordingly, if the UE supports a plurality of CCs, data can be transmitted or received to/from the plurality of CCs corresponding to a plurality of serving cells.
Spectrum aggregation (bandwidth aggregation or carrier aggregation) indicates supporting a plurality of CCs. The spectrum aggregation is implemented so as to support throughput increase, to prevent cost increase due to a wideband radio frequency (RF) device, and to ensure compatibility with the conventional system.
FIG. 5 illustrates one example of a multiple carrier. The multiple carrier includes five component carriers (CC #1, CC #2, CC #3, CC #4 and CC #5), and each CC has a bandwidth of 20 MHz. Accordingly, if five CCs are allocated in a granular manner of a CC having a bandwidth of 20 MHz, a maximum bandwidth of 100 Mhz can be supported.
The bandwidth or the number of the CC is merely exemplary. Accordingly, each CC may have a different bandwidth, and the number of downlink CCs may be equal to or different from the number of uplink CCs.
FIG. 6 illustrates a structure of a second layer of a base station for a multiple carrier, and FIG. 7 illustrates a structure of a second layer of a user equipment for a multiple carrier.
A MAC layer may manage one or more CCs. One MAC layer includes one or more HARQ entities. One HARQ entity executes hybrid automatic repeat request (Hybrid ARQ or HARQ) with respect to one CC. Each HARQ entity independently processes a transport block on a transport channel. Accordingly, a plurality of HARQ entities may transmit or receive a plurality of transport blocks on a plurality of CCs.
In the conventional art, when quality deterioration or failure of any downlink CC has been detected by the UE, the failure of the downlink CC may propagate (or influence) to (on) an uplink CC. This may cause the UE or the base station to determine that unnecessary radio link failure has occurred. Furthermore, the UE executes an RRC connection re-establishment procedure for recovery of the radio link failure, resulting in waste of radio resources.